Remove host argument:

Currently the idea is to later implement a -h (or -t) option to the
command, and/or a `target' recipe keyword. Another option might be to
use the target/host key when the key/value registry will be implemented
and made available to the recipe and task DSLs.

features/cli/usage.feature

@@ -5,5 +5,5 @@ Feature: CLI usage
     Then the exit status must be 64
     And the output must contain exactly:
     """
-    Usage: producer host recipe_file
+    Usage: producer recipe_file
     """